The Outlook database is in a *.pst file. You probably have two, one is the active and the other is the archive.

Search for them and take a look at their size. I don't know if there is still an issue with the newest versions, but there at least used to be issues with the files getting corrupted when they got up to a certain range in size. (2GB? I don't recall exactly.)

You can go into Oulook options (tools > options) and select the mail setup tab, then data files, and there you will see the files. Highlight one, select settings, and then compact. Do same for any others. Does that reduce the file size significantly?

My other suggestion is to try creating a new pst file, and see if there's any trouble sync'ing to the new file. If all is well, try copying some data from the old file and see if it makes any diference.

You may want to keep the old pst in Outlook so you can have it available. It won't sync with the blackberry, but it will be there as a reference when you are working with Outlook.